Short-term immobilization of ammonium and nitrate added to a grassland soil

Short-term immobilization of added 15NH+4-N and 15 NO−3-N (with or without added glucose) was determined at time intervals in a pre-conditioned grassland soil over laboratory incubation periods of up to 15 days at 25 C. Exchangeable N was removed from soil sub-samples by repeated 2 m KCl extraction and centrifugation. The solid residues were immediately blast frozen, freeze-dried and analysed for total N and N isotope ratio. Sub-samples without glucose amendment showed very little immobilization of added N (5 and <2% of added NH+4-N and NO−3-N respectively after 15 days). In contrast, when glucose was added, 16% of added NH+4-N and 8% of added NO−3-N appeared to be immobilized, and most immobilization occurred within about 3 days of N application. This preliminary experiment indicates that this simple method could be used routinely to compare the N immobilization potentials of different soils.